Process for grinding etching plates made of magnesium alloys for planographic printing



Patented June 14, 1932 ADOL'J BECK AND WALTER SCHMIDT, OF BITTERFELD, AND HUG-O DIBELKA, OF IDU'SSIE'L-v PATENT OFFICE DORF-OBERKASSEL, GERMANY,ASSIGNORS TO I. G. FARBENINDUSTBIE AKTIIEZNGE- SmLSCHAFT, FFRANKFORT-ON-THE-MAIN, GERMANY, A. CORPORATION OF GER- PB-OCESS FORGRINDING- ETCEING- PLATES MADE OF MAGNESIUM ALLOYS FOR PLANOGRAZPHICPRINTING No Drawing. Application filed. July 20, 1929, Serial No;.},379,870, and in Germany July 25, 1928.

This invention relates to a process for treating etching plates made ofmagnesium a1- loys for planographic or lithographic plates.

The metal plates for planographic printing are supplied to the user in ahighly polished condition to ensure that they are perfectly suitable foruse. The sensitized printing film does not easily adhere to such highlypolished surfaces, and it is therefore necessary to reduce the polish tosome extent and sometimes also to remove a certain fatty film. In thecase of zinc and copper plates, this is usually effected by grindingthem with a suspension of fine cleansing powders, such as emery,levigated chalk or the like, in water; and also, in many instances, by aslight preliminary etching the plates being treated, for a short time,with a very weak aqueous solution of nitric acid. When metal platescomposed of magnesium alloys are used, it has been observed that, inthis treatment, involving the application of aqueous solutions orsuspensions, hydrogen bubbles are disengaged on the surface of theplates and become 1mprisoned by the copying layer, in consequence ofwhich troubles arise in printing.

As a result of experiments it has now been ascertained that, with anytype of water encountered in practice, the plates can be etched, withcomplete suppression of the liberation of hydrogen due to a kind ofetching effect of the water, if the cleansing powders used in thetreatment are mixed with bichromate. The bichromate may be present inthese mixtures in excess of the quantity corresponding to its solubilityin water. The rinsing operation is also preferably performed with watercontaining bichromate, for instance, 1% of ammoniumbichromate. Afterthis preliminary treatment, the copying layer will be entirely free frombubbles.

Polishing compositions according to our invention may have, for example,the followihg compositions:

(a) 70 parts of levigated chalk and 120 parts of a 10% solution ofammoniumbichromate;

(6) 100 parts of finest emery powder and parts of potassiumbichromate.

(c) 30 parts of finest emery powder and 40 parts of levigated chalk aremade up to a pulp by thoroughly mixing with 65 parts of a 10% solutionof ammoniumbichromate.

Said standard-compositions may be made ready for use by adding water ina quantity sufiicient to make a solution containing at least about 0.3%of bichromate.

In comparison with the known process of using copying layers which areinert towards the metal by reason of their content of hichromate, thenew process constitutes an improvement in so far as it is based on thefurther discovery that the liberation of hydrogen is caused by thepreparatory treatment, and can therefore be prevented only by suitablymodifying said preparatory treatment.

We claim:

1. The process which comprises grinding etching plates made of magnesiumalloys for planographic printing with a grinding mediumin the presenceof a dilute aqueous solution of a bichroma-te.

2. The process which comprises grinding etching plates made of magnesiumalloys for p anographic printing with a grinding medium 1n the presenceof a dilute aqueous solution of a bichromate, and rinsing said lates inadilute aqueous solution of said bichromate. I

3. A composition for grinding etching plates of magnesium alloys,comprising a grinding medium and a water soluble bichromate.

4. A composition for grinding etching plates of magnesium alloys,comprising a grinding medium and a water soluble bichromate, thebichromate contents ranging from about 3 parts. to about parts to 100parts of said grindin medium.

5. A composltion for grinding etching plates of magnesium alloys,comprising a mixture of approximatel parts of emery powder and parts ofevigated chalk and 65 parts of a 10% aqueous solution of biehromate ofammonium.

In testimony whereof we have hereunto set our hands.

ADOLF BECK; WALTER SGHLHDT. HUGO DIBELKA.

